(1) Field of the Invention
The present invention relates to a method of checking virtual-pass-connection selection rights of fixed-length cell handling exchanges and to a method of checking and correcting virtual-pass-connection selection rights of fixed-length cell handling exchanges, both methods being suitable for use in checking whether or not an inconsistency is present in the setting of the selection rights of a plurality of adjacent exchanges for virtual path connections provided therebetween, in BISUP [Broadband ISDN (or B-ISDN) User's Part], which is the interexchange signal method for providing SVC (Switched Virtual Connection) services through B-ISDN (Broadband Aspects of ISDN). Moreover, the present invention relates to fixed-length cell handling exchanges to which the above-described methods are applied.
(2) Description of the Related Art
Through B-ISDN, it is possible to provide SVC services for establishing VPC (virtual path connection) among a plurality of ATM (Asynchronous Transfer Mode) exchanges, as required. ITU-T recommendations define a BISUP (B-ISDN User's Part) as a standardized interexchange signal method for providing SVC services.
According to the requirements of BISUP, one or a plurality of VPCs are provided between a plurality of adjacent ATM exchanges in order to transfer data among them. For example, each VPC provided between two adjacent ATM exchanges is selected by only one of the ATM exchanges when call/connection is established between the two ATM exchanges.
In other words, when establishing call/connection between two ATM exchanges, each of the ATM exchanges can possess selection rights for (or capture) different VPCs from among the plurality of VPCs.
More specifically, according to the requirements of ITU-T recommendations, an ATM exchange having a larger SPC (Signaling Point Code)--which is an identification code for an exchange--(or a larger SPC number) has selection rights for VPCs having even identification numbers (VPCIs: VPC Identifiers), whereas an ATM exchange having a smaller SPC number has selection rights for VPCs having odd VPCI numbers.
When a certain ATM exchange (hereinafter often referred to simply as an "exchange") establishes call/connection with another exchange, one VPC is selected from VPCs for which the certain exchange has a selection right. If the VPCs for which the certain exchange has selection rights are all used, selection of the VPC is left to the other exchange.
In this way, when the two exchanges need to individually establish call/connection at the same time, they are prevented from selecting an identical VPC.
It is also possible to arbitrarily set a VPC selection right according to the agreement between two exchanges rather than according to the default requirements defined by the previously-described ITU-T recommendations or the like. For example, the VPC selection right may be set according to the VPCI number.
A maintenance technician sets a selection right for VPCs when an exchange is (newly or additionally) installed.
Specifically, with reference to the setting of the VPC selection rights of the other adjacent exchanges, the maintenance technician sets the VPC selection right of the thus-installed exchange such that it differs from (or is consistent with) the setting of the VPC selection rights of the other exchanges.
The details of the setting of VPC selection rights set by the maintenance technician are retained through use of a selection right availability table in which selection-right availability is set for each of VPCs for which the exchange has selection rights, and a route table which stores an aggregation of VPCs (or routes).
In contrast, N-ISDN (Narrowband Aspects of ISDN) adopts a method of providing one exchange with priority to select VPCs in lieu of the previously-described method of providing one of the two exchanges with a selection right for VPCs.
In the case where selection rights for VPCs are set according to the agreement between the two exchanges, the selection rights are set by operations of the maintenance technician. The maintenance technician sometimes sets the VPC selection rights of the two exchanges erroneously, thereby resulting in inconsistencies in the setting of the VPC selection rights of the two exchanges (or the setting of the selection right availability table or the route table).
Specifically, inconsistencies arise in the setting of the VPC selection rights in the cases; for example, where two exchanges are set such that both of them have selection rights for a certain VPC; where two exchanges are set such that neither has a selection right for any of the VPCs; or where two exchanges are set such that neither has a selection right for a certain VPC.
If two exchanges are set such that both of them have selection rights for a certain VPC, a collision of VPC selection occurs when the two exchanges attempt to establish call/connection individually through an identical VPC at the same time, thereby resulting in incomplete establishment of call/connection (the VPC cannot be used at this time).
With such inconsistent setting of the VPC selection rights, incomplete call/connection occur frequently, thereby rendering the VPC unusable for a relatively permanent period. As a result, the efficiency of use of the VPCs is deteriorated.
If two exchanges are set such that neither has a right to use any of the VPCs, it becomes impossible to establish call/connection between them.
As described above, if two exchanges are set such that both of them have selection rights for a certain VPC, the maintenance technician acknowledges a collision of selection of the VPC. If two exchanges are set such that neither has a selection right for any of the VPCs, the maintenance technician acknowledges an inability to establish call/connection between them. In either case, it is possible for the maintenance technician to acknowledge inconsistencies in the setting of VPC selection rights.
These inconsistencies in the setting of VPC selection rights are not acknowledged before the exchanges start offering services through the corresponding VPC. Therefore, it is impossible to find an inconsistency in the setting of selection rights for each VPC before the VPC is put into service.
In contrast, if two exchanges are set such that neither has a selection right for a certain VPC, the VPC remains unavailable between these two exchanges for a relatively permanent period. Therefore, the efficiency of use of VPCs is deteriorated, as well.
However, if two exchanges are set such that neither has a selection right for a certain VPC, the maintenance technician will not acknowledge that the VPC is unavailable. Consequently, it is impossible to find an inconsistency in the setting of VPC selection rights.
Even if an inconsistency in the setting of VPC selection rights is found after both exchanges have commenced use of VPCs, it is impossible to automatically correct the inconsistency in the setting of the VPC selection rights in the BISUP. Therefore, it is impossible to prevent the reduction in efficiency of use of VPCs.